Can I add an attribute to the main html tag with knitr?

Hi.

The html generated by knitr starts like this:

<html lang="en" xml:lang="en" >

However I'd like it to be like this:

<html lang="en" xml:lang="en" prefix="og: https://ogp.me/ns#">

Is that possible?

Are you refering to knitr only (knitr::knit()) or rmarkdown (rmarkdown::render()) ?

What is your input format ? How to you render your document ?